Investigation of the selectivity of maltoporin channels using mutant LamB proteins: mutations changing the maltodextrin binding site

How to Enter a Bacterium: Bacterial Porins and the Permeation of Antibiotics

Despite tremendous successes in the field of antibiotic discovery seen in the previous century, infectious diseases have remained a leading cause of death. More specifically, pathogenic Gram-negative bacteria have become a global threat due to their extraordinary ability to acquire resistance against any clinically available antibiotic, thus urging for the discovery of novel antibacterial agents. One major challenge is to design new antibiotics molecules able to rapidly penetrate Gram-negative bacteria in order to achieve a lethal intracellular drug accumulation. Protein channels in the outer membrane are known to form an entry route for many antibiotics into bacterial cells. Up until today, there has been a lack of simple experimental techniques to measure the antibiotic uptake and the local concentration in subcellular compartments. Hence, rules for translocation directly into the various Gram-negative bacteria via the outer membrane or via channels have remained elusive, hindering the design of new or the improvement of existing antibiotics. In this review, we will discuss the recent progress, both experimentally as well as computationally, in understanding the structure-function relationship of outer-membrane channels of Gram-negative pathogens, mainly focusing on the transport of antibiotics.

High-Resolution Imaging of Maltoporin LamB while Quantifying the Free-Energy Landscape and Asymmetry of Sugar Binding

Maltoporins are a family of membrane proteins that facilitate the diffusion of hydrophilic molecules and maltosaccharides across the outer membrane of Gram-negative bacteria. Two contradicting models propose the sugar binding, uptake, and transport by maltoporins to be either symmetric or asymmetric. Here, we address this contradiction and introduce force-distance-based atomic force microscopy to image single maltoporin LamB trimers in the membrane at sub-nanometer resolution and simultaneously quantify the binding of different malto-oligosaccharides. We assay subtle differences of the binding free-energy landscape of maltotriose, maltotetraose, and maltopentaose, which quantifies how binding strength and affinity increase with the malto-oligosaccharide chain length. The ligand-binding parameters change considerably by mutating the extracellular loop 3, which folds into and constricts the transmembrane pore of LamB. By recording LamB topographs and structurally mapping binding events at sub-nanometer resolution, we observe LamB to preferentially bind maltodextrin from the periplasmic side, which shows sugar binding and uptake to be asymmetric. The study introduces atomic force microscopy as an analytical nanoscopic tool that can differentiate among the factors modulating and models describing the binding and uptake of substrates by membrane proteins.

Role of Electroosmosis in the Permeation of Neutral Molecules: CymA and Cyclodextrin as an Example

To quantify the flow of small uncharged molecules into and across nanopores, one often uses ion currents. The respective ion-current fluctuations caused by the presence of the analyte make it possible to draw some conclusions about the direction and magnitude of the analyte flow. However, often this flow appears to be asymmetric with respect to the applied voltage. As a possible reason for this asymmetry, we identified the electroosmotic flow (EOF), which is the water transport associated with ions driven by the external transmembrane voltage. As an example, we quantify the contribution of the EOF through a nanopore by investigating the permeation of α-cyclodextrin through CymA, a cyclodextrin-specific channel from Klebsiella oxytoca. To understand the results from electrophysiology on a molecular level, all-atom molecular dynamics simulations are used to detail the effect of the EOF on substrate entry to and exit from a CymA channel in which the N-terminus has been deleted. The combined experimental and computational results strongly suggest that one needs to account for the significant contribution of the EOF when analyzing the penetration of cyclodextrins through the CymA pore. This example study at the same time points to the more general finding that the EOF needs to be considered in translocation studies of neutral molecules and, at least in many cases, should be able to help in discriminating between translocation and binding events.

Structural Insights into Outer Membrane Permeability of Acinetobacter baumannii

Bacterial resistance against antibiotics is an increasing global health problem. In Gram-negative bacteria the low permeability of the outer membrane (OM) is a major factor contributing to resistance, making it important to understand channel-mediated small-molecule passage of the OM. Acinetobacter baumannii has five Occ (OM carboxylate channel) proteins, which collectively are of major importance for the entry of small molecules. To improve our understanding of the OM permeability of A. baumannii, we present here the X-ray crystal structures of four Occ proteins, renamed OccAB1 to OccAB4. In addition we have carried out a biochemical and biophysical characterization using electrophysiology and liposome swelling experiments, providing information on substrate specificities. We identify OccAB1 as having the largest pore of the Occ proteins with corresponding high rates of small-molecule uptake, and we suggest that the future design of efficient antibiotics should focus on scaffolds that can permeate efficiently through the OccAB1 channel.

Structural basis for outer membrane sugar uptake in pseudomonads

Substrate-specific outer membrane channels of gram-negative bacteria mediate uptake of many small molecules, including carbohydrates. The mechanism of sugar uptake by enterobacterial channels, such as Escherichia coli LamB (maltoporin), has been characterized in great detail. In pseudomonads and related organisms, sugar uptake is not mediated by LamB but by OprB channels. Beyond the notion that OprB channels seem to prefer monosaccharides as substrates, very little is known about OprB-mediated sugar uptake. Here I report the X-ray crystal structure of an OprB channel from Pseudomonas putida F1. The structure shows that OprB forms a monomeric, 16-stranded β-barrel with a constriction formed by extracellular loops L2 and L3. The side chains of two highly conserved arginine residues (Arg(83) and Arg(110)) and a conserved glutamate (Glu(106)) line the channel constriction and interact with a bound glucose molecule. Liposome swelling uptake assays show a strong preference for monosaccharide transport over disaccharides. Moreover, substrates with a net negative charge are disfavored by the channel, probably due to the negatively charged character of the constriction. The architecture of the eyelet and the absence of a greasy slide provide an explanation for the observed specificity of OprB for monosaccharides rather than the oligosaccharides preferred by LamB and related enterobacterial channels.

Hexose/Pentose and Hexitol/Pentitol Metabolism

Escherichia coli and Salmonella enterica serovar Typhimurium exhibit a remarkable versatility in the usage of different sugars as the sole source of carbon and energy, reflecting their ability to make use of the digested meals of mammalia and of the ample offerings in the wild. Degradation of sugars starts with their energy-dependent uptake through the cytoplasmic membrane and is carried on further by specific enzymes in the cytoplasm, destined finally for degradation in central metabolic pathways. As variant as the different sugars are, the biochemical strategies to act on them are few. They include phosphorylation, keto-enol isomerization, oxido/reductions, and aldol cleavage. The catabolic repertoire for using carbohydrate sources is largely the same in E. coli and in serovar Typhimurium. Nonetheless, significant differences are found, even among the strains and substrains of each species. We have grouped the sugars to be discussed according to their first step in metabolism, which is their active transport, and follow their path to glycolysis, catalyzed by the sugar-specific enzymes. We will first discuss the phosphotransferase system (PTS) sugars, then the sugars transported by ATP-binding cassette (ABC) transporters, followed by those that are taken up via proton motive force (PMF)-dependent transporters. We have focused on the catabolism and pathway regulation of hexose and pentose monosaccharides as well as the corresponding sugar alcohols but have also included disaccharides and simple glycosides while excluding polysaccharide catabolism, except for maltodextrins.

Site-directed mutagenesis of tyrosine 118 within the central constriction site of the LamB (maltoporin) channel of Escherichia coli. II. Effect on maltose and maltooligosaccharide binding kinetics

The 3-D structure of the maltooligosaccharide-specific LamB channel of Escherichia coli (also called maltoporin) is known from x-ray crystallography. The central constriction of the channel formed by the external loop 3 is controlled by tyrosine 118. Y118 was replaced by site-directed mutagenesis by 10 other amino acids (alanine (A), isoleucine (I), asparagine (N), serine (S), cysteine (C), aspartic acid (D), arginine (R), histidine (H), phenylalanine (F), and tryptophan (W)) including neutral ones, negatively and positively charged amino acids to study the effect of their size, their hydrophobicity index, and their charge on maltose and maltooligosaccharide binding to LamB. The mutants were reconstituted into lipid bilayer membranes and the stability constants for binding of maltose, maltotriose, maltopentaose, and maltoheptaose to the channel were measured using titration experiments. The mutation of Y118 to any other non-aromatic amino acid led to a substantial decrease of the stability constant of binding by factors between about two and six. The highest effect was observed for the mutant Y118A. Replacement of Y118 by the two other aromatic amino acids, phenylalanine (F) and tryptophan (W), resulted in a substantial increase of the stability constant maximally by a factor of almost 400 for the Y118W mutant. The carbohydrate-induced block of the channel function was used for the study of current noise through the different mutant LamB channels. The analysis of the power density spectra allowed the evaluation of the on- and off-rate constants (k(1) and k(-1)) of sugar binding. The results suggest that both rate constants were affected by the mutations. For most mutants, with the exception of Y118F and Y118W, k(1) decreased and k(-1) increased, whereas the opposite was found for the aromatic amino acid mutants. The results suggest that tyrosine 118 has a crucial effect on carbohydrate transport through LamB.

Sugar transport through maltoporin of Escherichia coli: role of the greasy slide

The lining of the maltodextrin-specific maltoporin (LamB) channel exhibits a string of aromatic residues, the greasy slide, part of which has been shown previously by crystallography to be involved in substrate binding. To probe the functional role of the greasy slide, alanine scanning mutagenesis has been performed on the six greasy slide residues and Y118 at the channel constriction. The mutants were characterized by an in vivo uptake assay and sugar-induced-current-noise analysis. Crystallographic analysis of the W74A mutant showed no perturbation of the structure. All mutants showed considerably decreased maltose uptake rates in vivo (<10% of the wild-type value), indicating the functional importance of the investigated residues. Substitutions at the channel center revealed appreciably increased (up to 100-fold) in vitro half-saturation concentrations for maltotriose and maltohexaose binding to the channel. Sugar association rates, however, were significantly affected also by the mutations at either end of the slide (W74A, W358A, and F227A), an effect which became most apparent upon nonsymmetrical sugar addition. The kinetic data are discussed on the basis of an asymmetric one-site two-barrier model, which suggests that, at low substrate concentrations, as are found under physiological conditions, only the heights of the extracellular and periplasmic barriers, which are reduced by the presence of the greasy slide, determine the efficiency of this facilitated diffusion channel.

Mathematical treatment of transport data of bacterial transport systems to estimate limitation in diffusion through the outer membrane

Bacterial transport systems are traditionally treated as enzymes exhibiting a saturable binding site giving rise to an apparent K(m)of transport, whereas the maximal rate of transport is regarded equivalent to the V(max)of enzymatic reactions. Thus, the Michaelis-Menten theory is usually applied in the analysis of transport data and K(m)and V(max)are derived from the treatment of data obtained from the rate of transport at varying substrate concentrations. Such an analysis tacitly assumes that the substrate recognition site of the transport system is freely accessible to substrate. However, this is not always the case. In systems endowed with high affinity in the micro M range or those recognizing large substrates or those exhibiting high V(max), the diffusion through the outer membrane may become rate determining, particularly at low external substrate concentrations. In such a situation the dependence of the overall rate of transport (from the medium into the cytoplasm) on the substrate concentration in the medium will no longer follow Michaelis-Menten kinetics. By analysing the deviation of transport data from the corresponding ideal Michaelis-Menten plot we developed a method that allows us to determine diffusion limitation through the outer membrane. The method allows us to find the correct K(m)of the transport system functioning at the inner membrane even under conditions of strong diffusion limitation through the outer membrane. The model was tested and validified with the Escherichia coli binding protein-dependent ABC transporter for maltose. The corresponding systems for sn -glycerol-3-phospate of Escherichia coli and the alpha -cyclodextrin transport of Klebsiella oxitoca were used as test systems.

In vivo and in vitro studies of transmembrane beta-strand deletion, insertion or substitution mutants of the Escherichia coli K-12 maltoporin

LamB of Escherichia coli K12, also called maltoporin, is an outer membrane protein, which specifically facilitates the diffusion of maltose and maltodextrin through the bacterial outer membrane. Each monomer is composed of an 18-stranded antiparallel beta-barrel. In the present work, on the basis of the known X-ray structure of LamB, the effects of modifications of the beta-barrel domain of maltoporin were studied in vivo and in vitro. We show that: (i) the substitution of the pair of strands beta13-beta14 of the E. coli maltoporin with the corresponding pair of strands from the functionally related maltoporin of Salmonella typhimurium yielded a protein active in vivo and in vitro; and (ii) the removal of one pair of beta-strands (deletion beta13-beta14) from the E. coli maltoporin, or its replacement by a pair of strands from the general porin OmpF of E. coli, leads to recombinant proteins that lost in vivo maltoporin activities but still kept channel formation and carbohydrate binding in vitro. We also inserted into deletion beta13-beta14 the portion of the E. coli LamB protein comprising strands beta13 to beta16. This resulted in a protein expected to have 20 beta-strands and which completely lost all LamB-specific activities in vivo and in vitro.

In vivo and in vitro studies of major surface loop deletion mutants of the Escherichia coli K-12 maltoporin: contribution to maltose and maltooligosaccharide transport and binding

The trimeric protein LamB of Escherichia coli K-12 (maltoporin) specifically facilitates the diffusion of maltose and maltooligosaccharides through the outer membrane. Each monomer consists of an 18-stranded antiparallel beta-barrel with nine surface loops (L1 to L9). The effects on transport and binding of the deletion of some of the surface loops or of combinations of several of them were studied in vivo and in vitro. In vivo, single-, DeltaL4, DeltaL5, DeltaL6, and double-loop deletions, DeltaL4 + DeltaL5 and DeltaL5 + DeltaL6, abolished maltoporin functions, but not the double deletion DeltaL4 + DeltaL6 and the triple deletion DeltaL4 + DeltaL5 + DeltaL6. While deletion of the central variable portion of loop L9 (DeltaL9v) affected maltoporin function only moderately, the combination of DeltaL9v with the double deletion of loops L4 and L6 (triple deletion DeltaL4 + DeltaL6 + DeltaL9v) strongly impaired maltoporin function and resulted in sensitivity to large hydrophilic antibiotics without change in channel size as measured in vitro. In vitro, the carbohydrate-binding properties of the different loop mutants were studied in titration experiments using the asymmetric and symmetric addition of the mutant porins and of the carbohydrates to one or both sides of the lipid bilayer membranes. The deletion of loop L9v alone (LamBDeltaL9v), of two loops L4 and L6 (LamBDeltaL4 + DeltaL6), of three loops L4, L5 and L6 (LamBDeltaL4 + DeltaL5 + DeltaL6) or of L4, L6 and L9v (LamBDeltaL4 + DeltaL6 + DeltaL9v) had relatively little influence on the carbohydrate-binding properties of the mutant channels, and they had approximately similar binding properties for carbohydrate addition to both sides compared with only one side. The deletion of one of the loops L4 (LamBDeltaL4) or L6 (LamBDeltaL6) resulted in an asymmetric carbohydrate binding. The in vivo and in vitro results, together with those of the purification across the starch column, suggest that maltooligosaccharides enter the LamB channel from the cell surface side with the non-reducing end in advance. The absence of some of the loops leads to obstruction of the channel from the outside, which results in a considerable difference in the on-rate of carbohydrate binding from the extracellular side compared with that from the periplasmic side.

Maltose/maltodextrin system of Escherichia coli: transport, metabolism, and regulation

The maltose system of Escherichia coli offers an unusually rich set of enzymes, transporters, and regulators as objects of study. This system is responsible for the uptake and metabolism of glucose polymers (maltodextrins), which must be a preferred class of nutrients for E. coli in both mammalian hosts and in the environment. Because the metabolism of glucose polymers must be coordinated with both the anabolic and catabolic uses of glucose and glycogen, an intricate set of regulatory mechanisms controls the expression of mal genes, the activity of the maltose transporter, and the activities of the maltose/maltodextrin catabolic enzymes. The ease of isolating many of the mal gene products has contributed greatly to the understanding of the structures and functions of several classes of proteins. Not only was the outer membrane maltoporin, LamB, or the phage lambda receptor, the first virus receptor to be isolated, but also its three-dimensional structure, together with extensive knowledge of functional sites for ligand binding as well as for phage lambda binding, has led to a relatively complete description of this sugar-specific aqueous channel. The periplasmic maltose binding protein (MBP) has been studied with respect to its role in both maltose transport and maltose taxis. Again, the combination of structural and functional information has led to a significant understanding of how this soluble receptor participates in signaling the presence of sugar to the chemosensory apparatus as well as how it participates in sugar transport. The maltose transporter belongs to the ATP binding cassette family, and although its structure is not yet known at atomic resolution, there is some insight into the structures of several functional sites, including those that are involved in interactions with MBP and recognition of substrates and ATP. A particularly astonishing discovery is the direct participation of the transporter in transcriptional control of the mal regulon. The MalT protein activates transcription at all mal promoters. A subset also requires the cyclic AMP receptor protein for transcription. The MalT protein requires maltotriose and ATP as ligands for binding to a dodecanucleotide MalT box that appears in multiple copies upstream of all mal promoters. Recent data indicate that the ATP binding cassette transporter subunit MalK can directly inhibit MalT when the transporter is inactive due to the absence of substrate. Despite this wealth of knowledge, there are still basic issues that require clarification concerning the mechanism of MalT-mediated activation, repression by the transporter, biosynthesis and assembly of the outer membrane and inner membrane transporter proteins, and interrelationships between the mal enzymes and those of glucose and glycogen metabolism.

Structure of the sucrose-specific porin ScrY from Salmonella typhimurium and its complex with sucrose

The X-ray structure of a sucrose-specific porin (ScrY) from Salmonella typhimurium has been determined by multiple isomorphous replacement at 2.4 A resolution both in its uncomplexed form and with bound sucrose. ScrY is a noncrystallographic trimer of identical subunits, each with 413 structurally well-defined amino acids. A monomer is built up of 18 anti-parallel beta-strands surrounding a hydrophilic pore, with a topology closely similar to that of maltoporin. Two non-overlapping sucrose-binding sites were identified in difference Fourier maps. The higher permeability for sucrose of ScrY as compared to maltoporin is mainly accounted for by differences in their pore-lining residues.

Identification of a new porin, RafY, encoded by raffinose plasmid pRSD2 of Escherichia coli

The conjugative plasmid pRSD2 carries a raf operon that encodes a peripheral raffinose metabolic pathway in enterobacteria. In addition to the previously known raf genes, we identified another gene, rafY, which in Escherichia coli codes for an outer membrane protein (molecular mass, 53 kDa) similar in function to the known glycoporins LamB (maltoporin) and ScrY (sucrose porin). Sequence comparisons with LamB and ScrY revealed no significant similarities; however, both lamB and scrY mutants are functionally complemented by RafY. Expressed from the tac promoter, RafY significantly increases the uptake rates for maltose, sucrose, and raffinose at low substrate concentrations; in particular it shifts the apparent K(m) for raffinose transport from 2 mM to 130 microM. Moreover, RafY permits diffusion of the tetrasaccharide stachyose and of maltodextrins up to maltoheptaose through the outer membrane of E. coli. A comparison of all three glycoporins in regard to their substrate selectivity revealed that both ScrY and RafY have a broad substrate range which includes alpha-galactosides while LamB seems to be restricted to malto-oligosaccharides. It supports growth only on maltodextrins but not, like the others, on raffinose and stachyose.

Crystal structures of various maltooligosaccharides bound to maltoporin reveal a specific sugar translocation pathway

BACKGROUND

Maltoporin (which is encoded by the lamB gene) facilitates the translocation of maltodextrins across the outer membrane of E. coli. In particular, it is indispensable for the transport of long maltooligosaccharides, as these do not pass through non-specific porins. An understanding of this intriguing capability requires elucidation of the structural basis.

RESULTS

The crystal structures of maltoporin in complex with maltose, maltotriose and maltohexaose reveal an extended binding site within the maltoporin channel. The maltooligosaccharides are in apolar van der Waals contact with the 'greasy slide', a hydrophobic path that is composed of aromatic residues and located at the channel lining. At the constriction of the channel the sugars are tightly surrounded by protein side chains and form an extensive hydrogen-bonding network with ionizable amino-acid residues.

CONCLUSION

Hydrophobic interactions with the greasy slide guide the sugar into and through the channel constriction. The glucosyl-binding subsites at the channel constriction confer stereospecificity to the channel along with the ability to scavenge substrate at low concentrations.

Combinatorial mutagenesis analysis of residues in the channel constriction loop L3 and neighbouring beta-strands in the LamB glycoporin of Escherichia coli

Members of the LamB family of sugar-selective porins (glycoporins) are beta-barrel proteins in the outer membrane of Gram-negative bacteria. To study the determinants of structure and sugar selectivity, 68 non-identical single amino acid substitutions were introduced into the stretch of sequence consisting of residues 106 through 125 in Escherichia coli LamB. This region includes all bar one residue of the channel constriction loop L3 and extends into the transmembrane beta 6 strand in the LamB structure. Mutants were assayed for dextrin utilization, starch binding, A binding, monoclonal antibody binding and for qualitative changes in protein expression. The importance of the L3 amino acids was emphasized by the observation that only four residues permitted a majority of neutral substitutions. Changes to the channel constriction zone strongly affected sugar binding yet no single amino acid change of residues exposed to the channel lumen caused a complete defect in maltodextrin utilization (i.e. were still Dex+). Substitutions in the L3 loop did not affect phage lambda binding, except one change at residue 122, nor changed recognition by anti-LamB antibodies specific for surface epitopes, consistent with the lack of a role of L3 residues in surface receptor function. In marked contrast, four substitutions in transmembrane strand beta 5 resulted in a Dex- phenotype and gross changes in protein properties, indicating the significance of beta 5 in the architecture of LamB.

Derepression of LamB protein facilitates outer membrane permeation of carbohydrates into Escherichia coli under conditions of nutrient stress

The level of LamB protein in the outer membrane of Escherichia coli was derepressed in the absence of a known inducer (maltodextrins) under carbohydrate-limiting conditions in chemostats. LamB protein contributed to the ability of the bacteria to remove sugar from glucose-limited chemostats, and well-characterized lamB mutants with reduced stability constants for glucose were less growth competitive under glucose limitation than those with wild-type affinity. In turn, wild-type bacteria were less growth competitive than lamB mutants with enhanced sugar affinity. In contrast to an earlier report, we found that LamB- bacteria were less able to compete in carbohydrate-limited chemostats (with glucose, lactose, arabinose, or glycerol as the carbon and energy sources) when mixed with LamB+ bacteria. The transport Km for [14C]glucose was affected by the presence or affinity of LamB, but only in chemostat-grown bacteria, with their elevated LamB levels. The pattern of expression of LamB and the advantage it confers for growth on low concentrations of carbohydrates are consistent with a wider role in sugar permeation than simply maltosaccharide transport, and hence the well-known maltoporin activity of LamB is but one facet of its role as the general glycoporin of E. coli. A corollary of these findings is that OmpF/OmpC porins, present at high levels in carbon-limited bacteria, do not provide sufficient permeability to sugars or even glycerol to support high growth rates at low concentrations. Hence, the sugar-binding site of LamB protein is an important contributor to the permeability of the outer membrane to carbohydrates in habitats with low extracellular nutrient concentrations.

Induction of the lambda receptor is essential for effective uptake of trehalose in Escherichia coli

Trehalose transport in Escherichia coli after growth at low osmolarity is mediated by enzyme IITre of the phosphotransferase system (W. Boos, U. Ehmann, H. Forkl, W. Klein, M. Rimmele, and P. Postma, J. Bacteriol. 172:3450-3461, 1990). The apparent Km (16 microM) of trehalose uptake is low. Since trehalose is a good source of carbon and the apparent affinity of the uptake system is high, it was surprising that the disaccharide trehalose [O-alpha-D-glucosyl(1-1)-alpha-D-glucoside] has no problems diffusing through the outer membrane at high enough rates to allow full growth, particularly at low substrate concentrations. Here we show that induction of the maltose regulon is required for efficient utilization of trehalose. malT mutants that lack expression of all maltose genes, as well as lamB mutants that lack only the lambda receptor (maltoporin), still grow on trehalose at the usual high (10 mM) trehalose concentrations in agar plates, but they exhibit the half-maximal rate of trehalose uptake at concentrations that are 50-fold higher than in the wild-type (malT+) strain. The maltose system is induced by trehalose to about 30% of the fully induced level reached when grown in the presence of maltose in a malT+ strain or when grown on glycerol in a maltose-constitutive strain [malT(Con)]. The 30% level of maximal expression is sufficient for maximal trehalose utilization, since there is no difference in the concentration of trehalose required for the half-maximal rate of uptake in trehalose-grown strains with the wild-type gene (malT+) or with strains constitutive for the maltose system [malT(Con)]. In contrast, when the expression of the lambda receptor is reduced to less than 20% of the maximal level, trehalose uptake becomes less efficient. Induction of the maltose system by trehalose requires metabolism of trehalose. Mutants lacking amylotrehalase, the key enzyme in trehalose utilization, accumulate trehalose but do not induce the maltose system.

Combinatorial mutagenesis of the lamB gene: residues 41 through 43, which are conserved in Escherichia coli outer membrane proteins, are informationally important in maltoporin structure and function

A new strategy for combinatorial mutagenesis was developed and applied to residues 40 through 60 of LamB protein (maltoporin), with the aim of identifying amino acids important for LamB structure and function. The strategy involved a template containing a stop codon in the target sequence and a pool of random degenerate oligonucleotides covering the region. In vitro mutagenesis followed by selection for function (Dex+, ability to utilize dextrins) corrected the nonsense mutation and simultaneously forced incorporation of a random mutation(s) within the region. The relative importance of each residue within the target was indicated by the frequency and nature of neutral and deleterious mutations recovered at each position. Residues 41 through 43 in LamB accepted few neutral substitutions, whereas residues 55 through 57 were highly flexible in this regard. Consistent with this finding was that the majority of defective mutants were altered at residues 41 to 43. Characterization of these mutants indicated that the nature of residues 41 to 43 influenced the amount of stable protein in the outer membrane. These results, as well as the conserved nature of this stretch of residues among outer membrane proteins, suggest that residues 41 to 43 of LamB play an important role in the process of outer membrane localization.

Porin of Pseudomonas aeruginosa forms low conductance ion channel in planar lipid bilayers

Protein E1, a porin of the outer membrane of Pseudomonas aeruginosa, was reconstituted into planar lipid bilayers. Single channel conductance of the protein appeared to be 230 pS (pico siemens) in 1 M KCl-10 mM Hepes, pH7.2. This value is approximately 5 times lower than the conductance of the OmpF channel of Escherichia coli. Conductance increased linearly as the membrane potential was raised from -200 mV to +200 mV, and was nearly proportional to the KCl concentration. These results show that protein E1 is probably a genuine porin in the P. aeruginosa outer membrane supporting the earlier conclusion that protein E1 forms a small channel.